Loudspeakers in Vented Boxes: Part 1
An investigation of the equivalent circuits of loudspeakers in vented boxes shows that it is possible to make the low-frequency acoustic response equivalent to an ideal high-pass filter or as close an approximation as is desired. The simplifying assumptions appear justified in practice and the techniques involved are simple. The low-frequency performance of a loudspeaker can be adequately defined by three parameters, the resonant frequency f, a volume of air V, equivalent to its acoustic compliance, and the ratio of electrical resistance to motional reactance at the resonant frequency Q. From these three parameters, the electroacoustic efficiency (symbol) can be found also. A plea is made to loudspeaker manufacturers to publish these parameters as basic information on their product. The influence of other speaker constants on these parameters is investigated. When f and V are know, a loudspeaker box can be designed to give a variety of predictable responses which are different kinds of high-pass 24-dB per octave filters. For each response, a certain value of Q is required which depends not only on the Q of the loudspeaker but also the damping factor of the amplifier, for which a negative value is often required. The usual tuning arrangement leads to a response which can be that of a fourth-order Butterworth filter. This, however, is only a special case, and a whole family of responses may be obtained by varying the volume and tuning of the box. Also an empirical -law- may be overcome by the use of filtering in the associated amplifier. For example, for a given frequency response, the box volume can be reduced at the price of increased low-frequency output from the amplifier and vice versa, with little change in the motion required of the loudspeaker. Acoustic damping of the vent is shown to be unnecessary. Examples are given of typical parameters and enclosure designs.
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